Chair back



FIG. 1 is front elevation view of a chair back;

FIG. 2 is front perspective view of the chair back;

FIG. 3 is side elevation view of the chair back; and,

FIG. 4 is top plan view of the chair back.

The broken-line disclosure of the remainder of the chair is for illustrative purposes only and forms no part of the claimed design. 

The ornamental design for a chair back, as shown and described. 